Conventional power supply electric safety specifications require that input terminals of a power supply have no voltage when a power source for the power supply is shut down. In order to meet this requirement, power supply designers typically employ an “OR” diode in series with the input bus to prevent input storage capacitor charge to flow backward to the input terminals of the power supply. This may eliminate fire and electrical shock hazards. Additionally, an “OR” diode may also be utilized in redundant power supply systems at the outputs of parallel connected power supplies to prevent a power supply with a low output voltage from drawing current from a power supply with a higher output voltage or vice versa. An “OR” diode may refer to a plurality of diodes in which the cathodes of each diode are connected to a common output of a plurality of redundant power supplies and anodes of each diode are coupled to the individual outputs of the individual power supplies.
A problem associated with the use of a diode to provide isolation is the voltage drop associated with a diode. Typically, diodes create a 0.7 volt drop that causes a substantial power loss for the power supply system. One approach utilized to reduce power loss has been the use of a Schottky diode. Schottky diodes may reduce the voltage drop to a 0.3-0.4 volt drop. A further reduction of power loss may be achieved through use of a MOSFET body diode that is shunted when the MOSFET is turned on. This reduces the power dissipation to an amount of the drain to source current squared (Ids2) times the drain to source resistance of the MOSFET when on (RDSon) which is a fraction of the power dissipation of a diode. When an input voltage is present at the MOSFET, the MOSFET turns on, allowing current to flow. When an input voltage is not present, input storage capacitor charge may not flow backward to the input terminals of the power supply.
A drawback associated with MOSFET circuits based on input voltage sensing is the time delay between shutting down a voltage source and when the MOSFET has been switched off. This may allow the input voltage to persist at the input terminals of the power supply for several hundred milliseconds after the input supply has been shut off. Additionally, a current of an amount up to hundreds of amperes may exist for several milliseconds to the voltage source. Consequently, a method and system for providing improved isolation for power supplies while minimizing power dissipation is necessary.